In general, a transmission line refers to a conductor system consisting of several conductors, and employing a propagation operation of a wave by electrical parameters, which are distributed between conductors, for example, such as resistance, inductance, conductance, and capacitance per unit length.
Recently, active research has been conducted on methods of implementing a Left-Handed (LH) characteristic by employing this transmission line. The LH characteristic refers to a characteristic in which the propagation directions of an electric field, a magnetic field, and electromagnetic waves comply with Fleming's left hand rule contrary to Fleming's right hand rule, and is related with a theory of artificial “metamaterial.” The term “metamaterial” generally refers to a material, which is synthesized by an artificial method so as to exhibit special electromagnetic properties that can be seen rarely in the natural world.
A construction of the transmission line having the LH characteristic will be described below with reference to FIGS. 1 and 2. While a typical transmission line equivalent model is represented by an equivalent circuit of a serial inductor and a parallel capacitor, in a transmission line structure comprising a serial capacitor CL and a parallel inductor LL, in which the positions of the serial inductor and the parallel capacitor are exchanged, as illustrated in FIG. 1, there occurs a phenomenon in which the phase velocity of electromagnetic waves transmitted through the transmission line structure is reversed.
FIG. 1 shows an equivalent circuit of the transmission line having the serial capacitor and the parallel inductor. In this transmission line, when a phase velocity and a group velocity are calculated, a LH propagation characteristic is obtained in which the phase and group velocities are oriented in opposite directions.
Meanwhile, a more general structure in which a transmission line (hereinafter, referred to as a ‘RH transmission line’) representing a Right-Handed (RH) characteristic and a transmission line (hereinafter, referred to as a ‘LH transmission line’) representing a LH characteristic are integrated has been known as a transmission line (hereinafter, referred to as a ‘CRLH transmission line’) representing a Composite Right/Left Handed (CRLH) characteristic. An equivalent circuit of a CRLH transmission line is shown in FIG. 2.
The structure arranged as shown in FIG. 2 has the characteristic of the LH or RH transmission line depending on whether the influence of any one of the inductor and the capacitor of a serial connection unit and a parallel connection unit is significant in a specific frequency band.
The structure has a stopband characteristic at a resonant frequency of the serial unit and the parallel unit. This fact can be easily confirmed in the transmission characteristic of the general CRLH transmission line shown in FIG. 2. In more detail, at a low frequency band, the LH transmission characteristic mainly appears due to the action of a serial capacitor CL and a parallel inductor LL, whereas at a high frequency band, the RH transmission characteristic mainly appears due to the action of a serial inductor LR and a parallel capacitor CR. A stopband of electromagnetic waves exists between the two regions.
A construction of a transmission line in which the CRLH transmission line model is implemented actually will be described below with reference to FIG. 3.
In an actual implementation, each inductor and each capacitor can be implemented as a concentrated constant circuit by mounting a capacitive element and an inductive element of a Surface Mount Device (SMD) chip type or as distributed constant circuit by forming an IDT (interdigital) capacitive element and an inductive element on a circuit pattern.
FIG. 3 shows an example of a conventional CRLH transmission line constructed by forming an IDT capacitive element and an IDT inductive element on a circuit pattern.
The conventional transmission line largely includes capacitive elements 310, inductive elements 50 and a ground unit 30.
The capacitive elements 310 have an IDT pattern and are arranged at predetermined intervals in the length direction. The inductive elements 50 are formed on the same plane as that of the capacitive element 310, and have a stub shape projecting between the capacitive elements 310 in a lateral direction.
The ground unit 30 has a ground surface form provided on the other side of a substrate 1, and is electrically connected to one ends of the inductive elements by conductive connection elements 15. The connection elements 15 can be formed through via holes penetrating both surfaces of the substrate 1.
The serial capacitor CL FIG. 2 is formed by the capacitive element 310 having the IDT pattern, and the parallel inductor LL FIG. 2 is formed by the inductive element 50 whose ends are shorted.
A parasitic capacitive component between an IDT structure and a ground surface forms the parallel capacitor CR of FIG. 2. The serial inductor LR of FIG. 2 is formed by current existing on the IDT pattern and entire structure operates as the CRLH transmission line.
However, the above conventional transmission line has the following problems.
The value of the serial capacitor can vary by controlling an detailed shape of IDT, a distance between the elements and so on, but has many limitations in changing an inductance value in the inductor. In other words, in order to increase the inductance, the length of the inductive element projecting in a lateral direction on the same plane as that of the capacitive element must be increased. Accordingly, there was a problem in that the width of the substrate increases, resulting in an increase of the overall size of a device.
Meanwhile, unlike the above method, the inductive element can be formed from a conductive material formed in the via hole between the substrates. In this case, however, there was a problem in that the inductance value could not be changed according to a design condition since the width, material, etc. of the substrate are defined.